The mechanisms which determine whether an orofacial stimulus is perceived as painful or innocuous are undetermined. Most investigators have felt that these two types of stimuli are qualitatively different but recently, it has been suggested that they may be only quantitatively different. Both ideas are based on the sensory changes which follow the surgical procedure of trigeminal tractotomy. Many physiological studies of orofacial sensation have employed dental pulp stimulation as presumably painful stimulus. The effect of trigeminal tractotomy on dental sensation is unknown. It is therefore impossible to propose a unified physiological schema for the mechanisms by which dental and facial stimuli are distinguished. The proposed research will employ psychophysical, physiological, and pharmacological techniques to study dental and facial sensory perception in monkeys with chronically implanted dental electrodes, before and after trigeminal tractotomy. The fringe patterns of trigeminal neurons in response to identical stimuli will also be analyzed in acute experiments, after tractotomy. The study will help answer the crucial question as to the effect of trigeminal tractotomy on dental sensation and will allow correlation of sensory changes and neuronal firing patterns. This research is essential before a unified theory of the mechanism of dental and facial sensation can be proposed. A thorough understanding of the mechanisms of dental and facial pain is essential, before efficacious means can be devised for treating the many patients who suffer from intractable pain related to dental disease, cancer of the head and neck and idiopathic pain syndromes such as trigeminal neuralgia, post herpetic neuralgia, and anesthesia dolorosa.